there's probably 10,000 or more living creatures in just a handful of soil. one of the most common of all can hibernate in near space like temperatures for over 300 years... and come back to life with lower temps and the addition of water. That's just one of the most common forms of life here on earth that we are aware of. But it wouldn't even take a 'living' creature. There is a virus (a sexually transmitted disease i beleive) that isn't actually considered to be living, it doesn't contain a DNA sequence, only RNA, and when it come into contact with a cell body it invades the cell and the cell starts to produce more of the virus. Earth would only need those 'blank cells' created by the tidal and volcanic forces... and then have -one- of these 'non-living' messengers come into contact with it. But here's some info on that crazy asss creature i was talking about.... they look kind of like moles, but can pull in their limbs and exude a waxy outer shell... after which they are called a 'tun' and are nearly indestructible:
THE TARDIGRADE: NATURE'S MOST INDESTRUCTIBLE CREATURE
The tiny tardigrade, whose size is no larger than the head of a pin, has proved to be one of Nature's toughest survivors. Laboratory experimenters have immersed it in liquid helium down to a savage -272 degrees Celsius. They have left it at -192 degrees Celsius for 20 months, and cooked it for a week at 92 degrees Celsius in ether, alcohol and other noxious chemicals. Restored to normal temperature and given water, the tardigrade strolls away. Some specimens were brought back to life after 120 years in a dry and dusty museum.
Obviously the tardigrade has abilities human beings lack. Tardigrades dwell in mud, damp seashore sand, or on the water film surrounding the leaves of mosses and lichens. Some varieties are entirely aquatic. Zoologically they fall somewhere between worms and insects and can move about by wriggling on four pairs of stubby legs. There are tardigrades worldwide except in the tropics and Antarctica. A single gram of dried moss has been known to yield 22,000 of them.
"Tardigrades are
polyextremophiles and are able to survive in extreme environments that would kill almost any other animal. Some can survive temperatures close to
absolute zero[4], temperatures as high as 151 °C (303 °F), 1,000 times more radiation than any other animal
[5], nearly a decade without water, and even the vacuum of space.
[6]"